When a fuel containing sulfurous components is combusted, these components, with the exception of those that become fixed to the ash, are released into the atmosphere as sulfur dioxide (SO2). This gas has a harmful effect not only on humans and animals, but also on the environment, since it falls to earth as acid rain.
In the past, concern over sulfur dioxide has led to large-scale combustion equipment and plants being fitted with devices to desulfurize their exhaust smoke. Most of these apparatuses employ the wet gas method.
To desulfurize exhaust gas using the wet gas method, the exhaust gas is brought into contact with an absorption liquid containing an alkali such as lime. The SO2 is absorbed and removed. The sulfite generated in the absorption liquid when the SO2 from the exhaust gas is absorbed is oxidized to produce a more stable sulfate. Normally, this is accomplished by injecting air into the absorption liquid.
Various devices have been developed in the prior art to inject air into the absorption liquid. For example, in FIG. 13(A), air (10) is injected into every part of collection tank 102 by a large number of pipes 116 (Technique A). In FIG. 13(B), air (10) is injected through a rotating arrangement of pipes 115, as they rotate, into every part of collection tank 102 (Technique B). In FIG. 13(C), air which is injected through a number of fixed pipes 117 is mixed and perturbed by agitator 119 (Technique C). In FIG. 14, auxiliary pipe 118, which circulates the absorption liquid in collection tank 102, has a separate circulation pump 118b. On pipe 118 is another pipe, 118a, to inject the air. In pipe 118, a mixture of air and liquid is formed, and this mixture is sprayed into the tank. Agitator 119 is also used to disperse the air (10) through all parts of collection tank 102 (Technique D).
Technique E is another way to inject air into the absorption liquid. Instead of the independent pipe 118 used to circulate the absorption liquid in Technique D described above, a pipe 110a (See FIG. 10) is used which branches off from pipe 103, the pipe which sprinkles the absorption liquid toward the exhaust gas. This method does not feature an agitator 119.
Of the various means described above to inject air into the liquid, Technique B in FIG. 13(B) needs neither an independent pipe to circulate the liquid nor an agitator, and its oxidation capacity is high. However, it would be difficult to install such a system in an absorption tower, which is currently the most commonly used type of scrubber. The system shown in FIG. 13(A), in which a large number of fixed pipes 116 are used, is the only one possible. The limitations of the oxygenating capacity of this method require a tank of considerable size.
There is a strong demand for a solution to this problem realized by equipment which can be installed on the sidewalls of the tank, which is not limited to a single location, and which has the same oxygenating capacity as Technique B described above. A number of plans have been advanced for such equipment.
The agitator 119 shown in Techniques (C) and (D) is needed to maintain an efficient oxygenating capacity and to prevent the accumulation of sediments produced by oxygenation. However, the equipment required for such an agitator is expensive and bulky.
An example of an oxygenating apparatus in current use may be found in the Japanese Utility Model Publication (Koukai) 62-194423.
The oxygenating apparatus disclosed in that publication is in a wet gas desulfurizing apparatus for scrubbing sulfur from exhaust smoke. In it, the gas inlet unit by which exhaust gas is introduced is connected to the absorption tower, and a pipe to sprinkle the absorption liquid is connected to a sprinkler which is mounted in the tower above the collection tank, which sits on the floor of the tower. A pipe to circulate the absorption liquid branches off from the aforesaid sprinkler pipe and is connected to the collection tank on the floor of the tower. An air-blowing means to blow air into the tower is installed in the branch pipe.
The oxygenating means (i.e., the air-blowing device) described in this publication is actually designed as follows. (See FIG. 10.)
As can be seen in FIG. 10, this concept corresponds to the oxygenating apparatus described as Technique E above.
The absorption liquid in collection tank 102 on the floor of tower 101 is conducted through the exhaust gas conduit (not shown) by way of circulation pump 104 and pipe 103. The sprinkled liquid absorbs and dissolves the SO2 in the exhaust gas and is then returned to tank 102, where it accumulates.
In this method, the means of oxygenating the absorption liquid in tank 102 is the device to blow air described below.
Downstream from circulation pump 104 on pipe 103, the pipe which sprinkles the absorption liquid, is branch pipe 110a, which recirculates the liquid. One end of this pipe is connected to circulation pump 104; the other is connected to collection tank 102. Air pipe 105, which has a smaller diameter than the branch pipe 110a, is installed on that pipe. The end of the air pipe, 105a, is inserted into pipe 110a, and it is bent so that its axis is co-linear with the axis of pipe 110a. The airflow 10 which comes through the pipe travels in the same direction as the absorption liquid 11 flowing through pipe 110a, and it is discharged flowing downstream, having been turned by the end of the pipe. An air pump 106 is installed on the air pipe 105.
In addition to the oxygenating apparatus described above, various other designs have been suggested, including another means to blow air into the tower to promote oxidation of the absorption liquid, a means to use the airflow to break the liquid into small drops and a device which does not require the use of an agitator.
In the method described in Japanese Patent Publication (Koukai) 8-257347, which is illustrated in FIGS. 11(A) and (B), circulation pump 104 drives the absorption liquid out of the upper portion of collection tank 102, which sits on the floor of tower 101, through pipe 103. It is sprayed into the tower through nozzles (not shown). In the process of falling, the liquid comes in contact with the exhaust gas. The liquid, which now contains a high concentration of SO2 is conducted directly into collection tank 102 via pipe 110b. The upward current generated in tank 102 by the downward thrust of the downward-flowing current distributes the liquid and prevents the sedimentation of particulates such as gypsum.
The air used for oxidation may be blown into liquid 11 as it is being conduct ed to the bottom of tank 102, or it may be introduced in pipe 110b and brought into contact with the liquid containing a high concentration of SO2. This will speed up the oxidation reaction. The air can be distributed uniformly throughout tank 102 in the form of tiny bubbles which escape through pinholes 111a on the bottoms of a traylike array of pipes 111.
In the method just described, then, air is blown into the absorption liquid just after it has come in contact with the exhaust gas, when it contains a high concentration of SO2. This is the means to promote oxygenation of the absorption liquid. The air which is introduced speeds up the oxidation reaction. The means used to introduce the air in small quantities in order to bring about effective liquid-vapor contact is to blow the air into the liquid which is flowing by the force of gravity through the pipe which carries it back to the tank. The liquid-vapor mixture is discharged via pipe array 111 at the end of pipe 11. Array 111 consists of concentric rings with numerous pinholes on their underside. These pipes are installed over the entire surface of the bottom of the tank so that the air can be distributed throughout the tank.
However, sedimentation causes scale to form on the blowholes 111a, posing a serious problem with respect to maintenance.
Another design, shown in FIG. 12, renders unnecessary the agitator which creates flow in the liquid stored in the tank described above and so distributes the air to prevent the blowholes from becoming encrusted or sedimented with products such as gypsum. A number of jet nozzles 112 are oriented at a given angle with respect to the normal line of branch (recirculation) tank 102. The liquid in tank 102 is sprayed along the wall of the tank so that it is conveyed in the directions indicated by arrows A. Independent pipes 110c run between the bases of the jet nozzles 112 and tank 102. Air pipes 114 go into the nozzles forward of their bases. This configuration requires a great deal of space in which to mount the nozzles around the absorption tower which contains tank 102, and there is no way to downsize it. In addition, certain aspects of bubble formation and the diffusion of the bubbles into the liquid could stand improvement.
The apparatus shown in FIG. 10, in which air is injected into pipe 110a before the liquid is returned to the tank and distributed as tiny bubbles, has the following shortcomings.
First, because the air (10) is injected into the branch pipe under high pressure, a cavity of negative pressure is likely to form in the pipe. The pressure of the liquid will fluctuate and remain unstable, and the inner surfaces of the pipe are likely to erode. Second, the air bubbles which are dispersed through the pipe after the liquid and vapor have combined have to flow a considerable distance before they are uniformly distributed. Thus the pipe going from the point where the liquid and vapor meet to the collection tank must be fairly long. It would be very beneficial to reduce both the size of the equipment and its cost by solving this problem.